The impact of skin massage frequency on the intrafollicular transport of silica nanoparticles: Validation of the ratchet effect on an ex vivo porcine skin model

The human hair follicle (HF) represents a promising drug delivery target as an anatomical entity by itself, but also as a gateway enabling dermal or systemic bioavailability of active cosmetic and pharmaceutical ingredients. Due to its morphological characteristics, the HF provides a mechanically driven transport process of nanoparticles (NPs) when external forces are applied. This mechanism was presented as the so-called ratchet effect within the framework of an in silico study published recently. To investigate the influence of massage frequency on the penetration depth of NPs, and, by this, to validate the results obtained in silico, we implemented a corresponding application protocol on an ex vivo porcine skin model. In this connection, we compared three different skin massage frequencies (4.2 Hz, 50 Hz, 100 Hz) for the topical application of cyanine 5-labeled silica NPs (Cy5-SNPs). To elucidate the interplay of frequency and particle size, we furthermore applied Cy5-SNPs of three different diameters (300 nm, 676 nm, 1000 nm). Confocal laser scanning microscopy was utilized to investigate the follicular penetration depth of Cy5-SNPs on cryohistological slices. By this, we could demonstrate that the massage frequency and the follicular penetration depth exhibit an inverse relation pattern. Thus, the highest follicular penetration depth was observed within the 4.2 Hz group, while the lowest follicular penetration depth was found within the 100 Hz group for each Cy5-SNP size category. Additionally, we found that 676 nm Cy5-SNPs penetrated significantly deeper into HFs than 300 nm Cy5-SNPs and 1000 nm Cy5-SNPs, respectively. Summarizing, our results show that a low massage frequency including a dominant radial direction component leads to deeper follicular penetration depths of NPs than automated 3D-oscillation massage at 50 Hz or 100 Hz. Thus, our findings are in line with recent in silico results. Regarding translational purposes, our results are of high interest, since a massage executed at 250 BPM (4.2 Hz) is within a realizable range for manual application, e.g. for the implementation into clinical routines or the domestic use of drugs or cosmetics. Furthermore, the application of different massage frequencies offers the opportunity of patho-specific targeting as different anatomical parts of the HF can be reached.